Silver halide photosensitive materials are required to have, as basic performances, high sensitivity, fine granularity and low fog. Metal doping technique is one of the means capable of satisfying these needs. Metal doping is carried out in order to modify the physical properties of silver halide grain, thereby heightening the efficiency (quantum sensitivity) of changing photoelectrons to developable latent image. More specifically, metal doping technique is to introduce (dope) a simple metal ion or a metal complex containing a ligand into silver halide grain. By this metal doping, characteristics of silver halide grain are modified and the performance of the whole emulsion can be improved as is expected. Furthermore, the general and practical surface latent image type silver halide photographic emulsion is desired to have high contrast. The term “high contrast” as used herein means that the slope of the gradation part (linear part) of a characteristic curve drawn with an exposure amount on the abscissa and an optical density on the ordinate is steep. With regards to the gradation, it is important that the characteristic curve shows a steep rise at its toe (a region of an exposure amount in which optical density starts increasing). Even in a silver halide photosensitive material such as color printing paper, high contrast at a portion corresponding to the low density portion of the characteristic curve, in other words, a steep rise at its toe is desired in order to obtain a sharp and clear image.
In Research Disclosure No. 17643, item IA, described is metal ions or metal complexes which can be introduced into silver halide grains during formation by metal doping technique. Typical examples of a metal complex used at an initial stage of metal doping technique are metal complexes of platinum, palladium, iridium, rhodium and ruthenium as described in U.S. Pat. No. 2,448,060. These water soluble metal complexes, when used as a dopant, function as an antifoggant or stabilizer. In particular, hexa-coordinated metal complexes of palladium (IV) further exhibit sensitizing action. The complexes described in the patent have, as a ligand, a halide such as chloride or bromide. In U.S. Pat. No. 3,690,888, disclosed is a preparation process of a silver halide containing a polyvalent metal ion. This process comprises forming silver halide grain in the presence of a protective colloid composed mainly of an acrylic polymer. Examples of the polyvalent metal ion used here include bismuth, iridium, lead and osmium ions. The complex described in this patent has, in addition to the metal ion, a halide such as chloride or bromide as a ligand. The above-described documents disclose the effects available when a metal ion is incorporated in silver halide grain. With regards to dopants including cyanide ion, effects of them when incorporated in silver halide emulsion are described in Japanese Patent Publication No. 35373/1973, U.S. Pat. Nos. 3,790,390, 4,847,191, 4937180, and 4945035, and Japanese Patent (Application) Laid-Open Nos. 225445/1995, 20853/1990, 20854/1990, 20852/1990, 20855/1990, 118535/1991 and 118536/1991.
On the other hand, an iridium complex is used for reducing reciprocity (law) failure, particularly the high intensity reciprocity (law) failure. Doping of an iridium complex in silver halide grain is disclosed in Japanese Patent (Application) Laid-Open No. 285941/1989, 118583/1991, 213449/1992, 278940/1992, 66511/1993, 313277/1993, 82947/1994, 235995/1994, 72569/1995, 72576/1995, 202440/1999, 295841/1999, 227640/1999 and 267215/2000 and U.S. Pat. Nos. 4,933,272, 49337180 and 5037732. Fluoride ion, chloride ion, bromide ion, H2O, cyano, nitrosyl, thionitrosyl and oxalic acid ion are used as the ligand of the iridium complex.
It is known that use of a dopant for the purpose of attaining hard gradation (i.e., increasing contrast) or reducing the high intensity reciprocity (law) failure causes any one or combination of adverse effects such as sensitivity loss, latent image sensitization and reduction in maximum density. Iridium hexachloride (IrCl6−2, IrCl6−3) is mainly used at present for increasing contrast and reducing high intensity reciprocity (law) failure. This complex has high improving effects, but is accompanied with the problem that it causes all of the above-described adverse effects. It is a dopant limited in using amount and using method and thus, difficult to make full use of. A complex having a nitrosyl ligand is disclosed in European Patent Nos. 606893 and 606894, while a complex having a carbonyl ligand is disclosed in European Patent No. 415481. These complexes have effects as a dopant for increasing contrast. They however act as a deep electron trap so that even by the use of the compounds in specific examples in these documents, any of the above-described adverse effects cannot be decreased. In U.S. Pat. No. 5,360,712, a complex having a C—C, C—H or C—N—H bond is disclosed. Some of the specific examples of the compound described in it have high improving effects and adverse effects are reduced to some extent, but not a sufficient level. There is accordingly a demand for the development of a dopant further reduced in adverse effects.